1. Field of the Invention
The present invention relates to a control system for a manufacturing line or the like in a process plant, and, more particularly, to a control system capable of controlling a motor for driving a process machines in a process plant to correspond to changes in the running condition and the operational condition in the process.
2. Description of the Related Art
A process line such as a manufacturing line is specified by a rolling line, a physical distribution line and the like. A process line of this type is usually constituted in such a manner that it includes one or more process machines disposed along the line. Since the operational condition for the process machine closely relates to the change in the running condition or the like of the process line, the process machines must be collectively controlled by a process computer and/or a plant controller (hereinafter collectively called a "plant controller"). That is, a structure is required which is arranged in such a manner that a command is issued from the plant controller to each of the process machines in accordance with the change in the operational condition and the process machines are synchronously controlled in response to the above-described command to correspond to the change in the operational condition. For example, a process line for a rolling facility or an annealing facility in a rolling mill plant is constituted in such a manner that there are included a plurality of process machines such as rolled coil winders, milling rollers, bridle rolls, tension rolls, helper rolls and the like. In the process line thus-constituted, the speed of each of the motors for driving the corresponding process machines must be synchronously controlled to correspond to the change in the operational condition (such as the condition about the thickness of the rolled material). Hitherto, a structure has been employed in order to meet the above-described requirement, the structure being arranged in such a manner that a speed command is issued from the plant controller to motor controllers and each of the motor controllers controls the speed of the corresponding motor in accordance with the command thus-supplied. However, if the operational condition such as the thickness of the rolled material or the like is changed in the thus-arranged rolling process or the like, the inertia moment of the kinetic system of each of the motors or the like is changed. The above-described change leads to a fact that the rotational speeds of the motors become different from one another, causing the tension acting on the rolled material to be changed. Furthermore, the quality of the rolled material deteriorates and a problem of an excessive load trip arises in a portion of the motors. In order to prevent the above-described undesirable tension change, a necessity of establishing speed uniformity between the motors arises as an important factor.
Hitherto, in order to establish the above-described speed uniformity between motors, a method has been employed which is arranged in such a manner that each of the motors has, as the control characteristic thereof, a drooping characteristic, which corresponds to the load electric current, so as to absorb the set error or the like generated in the control system. Another method has been employed which is arranged in such a manner that the tension deviation is reduced by a tension control loop.
However, although a considerably satisfactory effect can be obtained from the above-described conventional methods in that the generated tension change can be reduced, there arises a problem in that the generation of the tension change cannot be prevented. A rolling line includes rolls such as the deflector rolls which must be rotated together with the material to be rolled. However, undesirable slip will take place between the roll and the material to be rolled or an excessively large load will act on the motor due to the change in the line speed or the tension change.
The above-described tension change can be prevented by adjusting the control gain such as the proportional gain of each of the motors and/or the control parameter such as an integral time constant in accordance with the change in the operational condition. Furthermore, the above-described slip and the excessively large load can be prevented by adjusting the limit value of the load current to be applied to the motor. However, any one of the conventional structures has not meet the above-described requirements. In addition, the above-described problems are taken place commonly to the other process lines, causing a problem to arise in that the change in the operational state of each of the process machines due to the change in the operation cannot be prevented.
On the other hand, if the setting of the control parameter for each of the motors is changed to correspond to the operational condition, a structure must be arranged in such a manner that the change in the operational condition in the process is detected by the plant controller, the control parameter for each of the motors is calculated in accordance with the above-described detection and the result of the calculation is transmitted to each of the motor controllers so that the setting of the control parameter is changed.
However, if a plurality of control parameters relating to the change in the setting are serial-transmitted by a conventional method (for example, a method disclosed in Japanese Patent Laid-Open No. 58-175991), the number of words to be transmitted increases by the quantity which corresponds to the plurality of the control parameters. In this case, the following problems take place in that: first, the receiving portion of the motor controller must have hardware such as serial-to-parallel converters and decoders of the number of words which correspond to the transmission format used to perform the serial transmission. Therefore, the overall structure becomes too complicated, causing the overall cost to be raised. Another problem arises in that, in a case where the number of the control parameters to be changed increases or the type of the control parameter is changed in accordance with the change in the control system, the structure cannot be easily changed to adapt to the above-described change.
What is even worse, the time required to complete the transmission operation becomes too long by the degree which corresponds to the increase in the number of words, causing the high speed response in the control operation to be deteriorated. That is, although the speed control system and the current control system must possess high speed response characteristics to correspond to the loading state which dynamically changes, the lengthened transmission time causes the period for transmitting the control command to be elongated and thereby the high speed response characteristic is deteriorated.